Deposition of silicon on a substrate by chemical vapor deposition (CVD) is practiced routinely in the semiconductor and tool industries. Atmospheric pressure CVD, low pressure CVD and plasma-assisted CVD are examples of commonly used processes. However, the high cost of capital equipment, i.e., the "reactor," and/or operation of the equipment result in high overall cost of the product and limit the commercial utility of CVD for depositing thick layers of silicon.
U.S. Pat. Nos. 5,227,195 and 4,582,560 to Sanjurjo, the disclosures of which are incorporated herein by reference, disclose a reactor design that takes advantage of the high heat and mass transfer rates of a fluidized bed reactor (FBR) and combines this with CVD chemistry. In particular, these patents teach the use of subhalide chemistry and a mode of feeding reactants in a condensed phase into an FBR-CVD reactor, a design that was well adapted to coat a substrate located inside the fluidized bed. However, when the same design is used to coat a substrate situated outside the bed, the gas species generated in the bed flows laminarly around the substrate creating a boundary layer as shown in FIG. 1. The formation of such a boundary layer limits the rate of coating deposition on a substrate located outside the bed.
The overall coating deposition rate at a given temperature may be increased by a combination of (a) enhancing the gas-to-solid mass transfer rate and (b) increasing the partial chemical potential gradient between the bed and the substrate surface. However, the overall rate of deposition is limited by the concentration gradient between the bulk of the gas phase as it flows out of the bed and past the substrate and the surface of the substrate.
For a typical substrate the equivalent boundary layer thickness will be of a magnitude similar to its size. Thus, for a 10 cm-diameter solar cell wafer, the thickness of the boundary layer could be 5 cm or greater for linear gas flow velocities typical in a conventional fluidized bed reactor, e.g., about 5 cm/s.
Locating the substrate near the top of the bed, e.g., approximately 1 mm from the bed, can significantly increase gas-to-solid mass transfer but it requires elaborate and expensive heat management designs and at best provides relatively slow deposition (1 micron per minute) of high quality epitaxial thin layers (typically 1 to 2 microns thick). For very fast deposition of polycrystalline silicon layers (4 microns per minute or greater) up to 100 .mu.m thick, a faster deposition approach would be desirable.